Re: Unrecoverable corruption after loss of cache

[Chris Murphy <lists@colorremedies.com>]

On Wed, Dec 4, 2019 at 5:34 PM Gard Vaaler <gardv@megacandy.net> wrote:
>
> > 4. des. 2019 kl. 22:09 skrev Chris Murphy <lists@colorremedies.com>:
> > There's a decent chance this is the cause of the problem. That kernel
> > does not have the fix for this bug:
> > https://www.spinics.net/lists/stable-commits/msg129532.html
> > https://bugzilla.redhat.com/show_bug.cgi?id=1751901
> >
> > As far as I'm aware the corruption isn't fixable. You might still be
> > able to mount the file system ro to get data out; if not then decent
> > chance you can extract data with btrfs restore, which is an offline
> > scraping tool, but it is a bit tedious to use.
> > https://btrfs.wiki.kernel.org/index.php/Restore
>
> That was my first thought too, but it seems too coincidental that I should happen across this bug at the same instant as my cache device failing. btrfs-restore doesn't like my filesystem either:

You know, I totally glossed over the cache device failing part of the
very first message 8-\ But yeah it would seem like the cache device
dropped a bunch of metadata. Really a lot more than I'd expect from
the aforementioned kernel bug. So chances are your suspicion is spot
on.

> > [liveuser@localhost-live btrfs-progs-5.4]$ sudo ./btrfs restore -Divvv /dev/bcache0 /mnt
> > This is a dry-run, no files are going to be restored
> > parent transid verify failed on 3719816445952 wanted 317513 found 313040
> > parent transid verify failed on 3719816445952 wanted 317513 found 308297
> > parent transid verify failed on 3719816445952 wanted 317513 found 313040
> > Ignoring transid failure
> > leaf parent key incorrect 3719816445952
> > Error searching -1

You might have to to try a lot of the btrfs-find-root block addresses
(start with highest transid working down) with btrfs restore -t option
to force it to use older roots. Maybe one of them will be intact. It's
also possible to isolate to a subvolume, if you have home on a
subvolume for example.

Unfortunately btrfs restore isn't a simple scraper, it doesn't
iterate. You have to do that part. It is tedious.


-- 
Chris Murphy